8-Core will not boot - Graphic card runs loud and oscolates (cycles)

I have a 2007 8-core MAC PRO. About two years ago, I got rid of the stock GPU and purchased an ATI Radeon HD4890 for it.

All was working well until the other night all of the sudden the fan became very loud and started oscolating (cycling) between loud and less loud. About every second was the pattern of sound.

The system will not boot now. No beeps, errors, etc. The only thing I see is that there are two small LED's for CPUA and CPUB that are on, along with a label by them that says "overtemp". Those are on. If I hit the little reset button by them, I can turn the led's off.

In diagnosing, I purshased another ATI 5770 single card slot and put that in. Still won't boot up. I can also hear a very faint ossolating sound with this card too, but it's definitely not as noticeable.

I've pulled out the card and ram and tried running it. No beeps, etc.

I'm thinking that it might be a bad PSU, but hate to buy one on speculation. Any other thoughts?

No. Thanks for mentioning that. I'm not as astute with Macs as I would like.

Tried reseting PRAM. No beeps, no nothing. Just out of curiosity, I removed one of the RAM slot cards. This time, the PRAM appeared to reset (chimed twice) and then went to the grey Apple screen. After that, it goes into an infinite reboot loop. Those two pesky red LED's come on the instant that it reboots for the first time. On the plus side, the fan on the old GPU seems to behave now, so it's definitely not that.

I then pulled the first RAM card and put in the second one. Same issue with infinite booting loop. So I put both RAM cards back in and the original issue reappears with no booting and a rapid cycling GPU fan.

RAM card #1 has 2-512mb, and 2-4GB modules

RAM card #2 has 2-512mb only.

I then put both RAM cards back in, but this time removed the two 512mb sticks from card #2. System wend into the infinite boot loop again.

I think you're right. I can boot up to the desktop now for at least a minute. Haven't tried longer than that until I do more of a cleaning. Seems that the CPU fins were rather clogged up. I blew them out which seemed to have helped. I'll do a more thorough cleaning today and see how long she stays running!

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It sounds like the computer is overheating (Or even worse, thinking it is overheating) Check that the system fans are all working, and maybe remove the CPU heatsinks, and clean them out. Also, replace the thermal compound on the processors if you can. Powermac G5s would act up if one of their processors died, but still show some signs of function. If CPUA works, but CPUB is damaged, you may be able to boot the computer with only CPUA installed. (I would only try this after ruling out any other problems) Unfortionatly, I'm not incredibly familiar with these machines. The RAM wouldn't all die at once, so that probably isn't your problem.